Devices known as exhaust brakes can be fitted into a vehicle's exhaust system and which, by generating a back pressure, can assist the vehicle in braking. Similar devices termed warm-up valves, can also assist in cab heating and in reducing the emission of unburned hydrocarbons by reducing the time for the engine to reach normal operating temperature. Unlike most large trucks, school buses and some smaller trucks do not have air compressors onboard since they are not equipped with air brakes. Therefore, it is desirable to have direct electric actuation of the exhaust valve.
Motor driven valves, per se, are known in the prior art. Conventional systems of this type, however, have a number of deficiencies. Prior art arrangements commonly employ a direct interconnection between the output shaft of the motor and the exhaust valve damper blade. Energization of the motor causes rotation of the output shaft and corresponding movement of the damper blade to a desired position (usually a closed position) relative to the exhaust system with which the damper blade is operatively associated. Typically, the motor remains energized to hold the damper blade against one or more stops within the exhaust system which properly position the damper blade relative thereto. When the motor is de-energized, the prior art approaches often employ a spring in operative association with the damper blade to return the damper blade to its "normal" (usually open) position relative to the exhaust system. Again, a stop arrangement is conventionally incorporated in the exhaust system to be engaged by the damper blade to maintain the damper blade in its position until the motor is once again energized.
Employment of the aforementioned spring return feature when there is a direct or positive interconnection between the motor and the damper blade causes difficulties. The spring return tends to over stress the motor when the blade hits the stop at normal condition. It has been found that the motor will "bounce" back and forth due to the inertia developed in the motor by the spring return. While the motor and damper blade eventually come to rest, the bounce action, especially over a period of time and frequent occurrence, causes considerable and undue wear of the motor's transmission gears, thereby shortening the life of the motor.
The present invention is directed to overcoming one or more of the problems set forth above.